I have a magician NPC I'm developing that's made a career out of hunting down toxic magicians. I figure she'd have a banishing focus for dealing with pesky toxic spirits. (Yes, I realise it's more efficient to blast the things with a stunbolt. Just humour me.

What I'm wondering is the following:
1. Is "toxic" considered a spirit type? If so, at what point does "toxic" supercede whatever other type the spirit is?
2. Does a "natural" Spirit Focus work equally well for toxic spirits as for "natural" Spirits? (For example, would a Beast Spirit Banishing Focus apply its bonuses against both beast spirits and abomination spirits?)
3. Is it reasonable to have a "Toxic Spirit Banishing Focus" that would give bonuses specifically against toxic forms of any given spirit, but not their "natural" forms?
4. Is there a compelling reason to say that you just can't apply spirit foci to toxic spirits, period? I know Street Magic says that only toxic magicians can summon or bind toxic spirits, but no such limitation exists for banishing, and I can't imagine there hasn't been at least one enchanter that's gone, "You need to banish toxic spirits? I bet I could make you somethin' for that!" (Various eco-policubs and Tír elves have probably sought to do just that.)
Thanks in advance.